Graphite monochromaters are highly oriented forms of high purity pyrolytic graphite which diffract x-rays and neutrons to generate a monochromatic beam of x-rays and/or neutrons for use in a spectrometer for measuring the characteristics of crystalline materials.
Graphite monochromaters are classified according to their neutron mosaic spread characteristic. Each known type of HOPG material will exhibit a different lattice spacing known to those skilled in the art as "d-spacing" for each effective neutron mosaic spread. The neutron reflectivity of graphite monochromaters should be high for application in neutron scattering instruments. At high Bragg angles, near 90.degree., the diffracted beam intensity is determined by the lattice spacing spread i.e. the d-spacing spread of the HOPG material in the monochromater. HOPG has a natural d-spacing spread which is large enough for high luminosity in back-reflection arrangements. Methods for further increasing this d-spacing spread would be of great value in providing increased neutron flux in backscattering instruments.
The mosaic spread is a measurement of the full width at half maximum intensity of the reflection of an x-ray beam from a sample of HOPG material when rotated through the Bragg angle to generate an x-ray diffraction curve known as a "rocking curve". The rocking curve is a graph of the intensity of the reflected x-rays as a function of the angular distance from a reference plane using Bragg's Law to determine the angular deviation. This calculation is made for each HOPG sample to permit its mosaic spread range to be measured so that each sample can be categorized into different standard mosaic spread ranges. U.S. Pat. No. 5,798,075, the disclosure of which is herein incorporated by reference, teaches how to assure a yield of up to 100% of HOPG material having a mosaic spread tailored to any desired preselected narrow range starting from HOPG processed material having a mosaic spread below the desired final mosaic spread specification for the material.
The present invention provides a method for increasing the effective d-spacing spread of an HOPG graphite monochromator by combining two or more conventional types of HOPG materials to form a composite HOPG structure formed of layers of separate HOPG materials each having a different average d-spacing and oriented with their layer planes parallel to one another. For example, an arbitarily selected HOPG grade or type material defined, for purposes of the present invention, as a Type 1 HOPG material, showed a higher average interlayer spacing and a greater lattice spacing spread i.e. d-spacing spread over the same mosaic range relative to a second more ordered HOPG graphite material arbitrarily defined as a Type 2 HOPG material to distinguish the two from each other. In neutron reflectivity measurements at 4.42 .ANG., the peak reflectivity of the more disordered type 1 HOPG material was found to be 5-10% higher than that of the more ordered grade Type 2 HOPG material, and the average interlayer spacing was .about.0.003 .ANG. higher. The d-spacing spreads of the more disordered HOPG grade were 0.08-12% and those of the more ordered grade were 0.03-0.07%. It was discovered in accordance with the present invention that by combining the two different grades of HOPG material to form a composite HOPG structure defined by the combination of the two different grades of HOPG material with each oriented relative to one another so that their layer planes were parallel that the effective d-spacing spread would increase to 0.15-0.18%. The increased .DELTA.d/d should yield higher neutron beam intensities in certain types of backscattering instruments.